In mobile telephony systems, apart from traffic channels on which speech and other types of data are transmitted between a base station and a mobile station, so called control channels transferring different types of control information are also used. Some of these control channels, like the traffic channels, transfer point-to-point information between the base station and the mobile stations. Other control channels are used by the base station for communication with all mobile stations within a sector cell at the same time. This requires an antenna at the base station having a sufficiently wide beam in the horizontal plane to cover the whole sector in question. Such a sector covering beam usually has a limited beam width in the vertical dimension and thus forms a horizontal disk, a so called flat beam.
The range requirement for channels for point-to-point information is the same as for channels for point-to-multipoint information. In present systems therefore one and the same sector antenna is used for both these functions. Point-to-point information, however, would not have to be transmitted from the base station in such a way that all mobile stations in the sector can receive it. It is enough that the mobile station for which the information is intended can. The base station, therefore, might concentrate the transmit power, even sideways, to the desired directions by using antennas having radiation patterns with narrow beams. If the same antennas are used for reception as well, a corresponding increase in the receiver sensitivity in the desired directions is achieved. This concentration of the transmit power and the receiver sensitivity can be used to increase the range and/or lower the power demands on the transmitters of both the base station and the mobile station. Since the channel frequency reuse spacing may be reduced with this method, the total capacity of the mobile telephony system may also be improved in this way.
One perceivable possibility of creating several simultaneous narrow beams is using a Butler matrix connected to an antenna array. A Butler matrix is a completely passive and reciprocal circuit comprising an interconnection of a number of hybrid couplers and either fixed phase shifting elements or transmission cables of varying lengths. A Butler matrix for an antenna of N elements, N being an integer number, usually a power of two, has N input ports and N output ports and therefore enables the generation of N narrow beams. A signal on one of the input ports to the Butler matrix results in signals on the output ports of the matrix of substantially the same amplitude but different phases. Each input port corresponds to a certain combination of phases on the output ports. Each one of these combinations generates a narrow beam from the antenna array. Since the antenna and the Butler matrix are completely reciprocal the system works as well for reception as for transmission.
Using an antenna fed from a Butler matrix, a set of narrow beams may be achieved, in which each individual radiation pattern has nulls for each angle at which another radiation pattern shows a maximum power (if the power is normalized using the antenna gain of the element pattern). Narrow beams meeting this criterion are said to be mutually orthogonal. Using a Butler matrix in combination with an antenna array to achieve a set of narrow beams is previously known per se.
It would be possible to use a separate sector antenna or alternatively one of the columns in an antenna array for the wide beam function. The lower antenna gain for the wide beam function would then have to be compensated with a higher amplifying power. The antenna gain here denotes the relationship between the maximum radiation of an antenna and the radiation of an ideal omnidirectional antenna with no loss, with the same supplied power. For example, an antenna array with eighth columns has an antenna gain that is 9 dB higher than a single antenna column or a sector antenna. This implies that the power amplification of the amplifier must be 9 dB higher to compensate for the lower antenna gain.
UK patent specification GB 2 169 453 discloses a method of generating a number of narrow beams with different directions and one wide beam covering the same area as all the narrow beams together using an antenna array. Here an electromagnetic lens of a so called Rotman type with parallel plates is used. On one side of the lens there are a number of beam ports and on the opposite side there are a number of antenna ports. Each one of these antenna ports is coupled, through an amplifying module, to an antenna element in an antenna array. Each beam port corresponds to one of the narrow beams in the prior art. Further, the lens is equipped with a separate connection, the position of which on the lens is adjusted so that the geometrical distances to the antenna ports cause the supplied signal power to this connection to be divided over the antenna ports in such a way that a wide beam is generated from the antenna array.
The electromagnetic lens is a spacious and expensive component that is not available on the market. Also, the wide beam, as in the previously described cases, obtains a lower antenna gain than the narrow beams, which requires expensive additional, separate amplification for the wide beam not to give a shorter range than that of the narrow beams.